Double key axial split-pin tumbler-type lock

ABSTRACT

An axial split-pin tumbler-type lock having a lock cylinder, a three-part plug assembly inserted in the cylinder, and a plurality of spring-pressed pin tumbler assemblies carried by the plug assembly is supplied with two keys, the first key serving to rotate two parts of the plug assembly conjointly to operate the lock, and the second key serving to rotate but one of the two parts, the one part being rotated to an inoperative position in which the lock cannot be picked directly and requiring for operation of the lock from that position the use of the second and first keys successively in that order.

United States Patent 1191 1111 3, Steinbach Nov. 4, 1975 DOUBLE KEY AXIAL SPLIT-PIN 3,729,964 5/1973 Hsu 70/338 TUMBLER.TYPE LOCK 3,875,773 4/1975 Thimot 70/337 [75] Inventor: Robert L. Steinbach, Chicago, Ill. Primary Examiner Robert L Wolfe [73] Assignee: Chicago Lock Co., Chicago, Ill. Attorney, g Firm-Norman Gel'lach [22] Filed: Feb. 11, 1975 [57] ABSTRACT Appl- N03 548,921 An axial split-pin tumbler-type lock having a lock cylinder, a three-part plug assembly inserted in the cylin- 52 us. (:1. 70/338; 70/339; 70/363 f a plurality of Spring-Pressed P "P P 51 Im. (:1. EB 27/08; EOSB /08 Sembhes earned by the plugfissembly SuPPlled [58] Field of Search /337, 338, 339, 340, keys, the first y F to mate two Parts Of 7O/341, 3 42, 343 362, 363, 382 385 the plug assembly con ointly to operate the lock, and the second key serving to rotate but one of the two 5 References Cited parts, the inehpglrt iaeirlzg rotated) to ankirzlogeratiye pod UNIT s1 ion 1n w 10 e 0c canno e plC e 1rec y an ED STATES PATENTS requiring for operation of the lock from that position i the use of the second and first keys successively in 9 3 a1 0 3,541,819 11/1970 Kerr that order 3,599,456 8/1971 Bessin 70/364 A 5 Claims, 10 Drawing Figures L a t E 14 1'. 63 4- 62 I 3 I I 94 38 0 92 1 46 as Ill US. Patent Nov. 4, 1975 Sheet 2 of2 3,916,657

I 1 861 13 as 50 90 92 54 56 25 4'1 2(\\\ r( V WILLA F DOUBLE KEY AXIAL SPLIT-PIN TUMBLER-TYPE LOCK BACKGROUND OF THE INVENTION This invention relates to an axial split-pin tumblertype lock and key combination employing two keys for operation of the lock.

Heretofore, various locks of the axial split-pin tumbler-type have been devised, having means designed to limit unauthorized operation of the locks. The prior locks have achieved varying degrees of success. Generally speaking, it is possible ultimately to pick any keyoperated lock, and efforts to prevent unauthorized picking are based upon rendering the operation difficult and time-consuming.

An example of a tamper-proof axial split-pin tumbler type lock is disclosed in US. Pat. No. 3,541,819 to Kerr. This patent discloses a lock constructed to induce a false picking, which does not operate the lock. A lock employing a plurality of keys for controlling operation of the lock is disclosed in US. Pat. No. 3,729,964 to Hsu.

It would be desirable to provide an axial split-pin tumbler-type lock and key combination having even lower vulnerability to picking than the combination of the aforesaid Kerr patent, and also having a relatively simple and economical comgination of members as compared, for example, to the lock of the aforesaid Hsu patent.

SUMMARY OF THE INVENTION In the invention, an axial split-pin tumbler-type lock having a lock cylinder, a three-part plug assembly inserted in the cylinder, and a plurality of spring-pressed pin tumbler assemblies carried by the plug assembly is supplied with two keys, the first key serving to rotate two parts of the plug assembly conjointly to operate the lock, and the second key serving to rotate but one of the two parts, the one part being rotated to an inoperative position in which the lock cannot be picked directly combination requiring for operation of the lock from that position the use of the second and first keys successively in that order. Two-key operation renders picking of the lock at least extremely difficult and time consuming and also serves to restrict access by key. Alternatively, if desired, the lock can be used with but the first key.

More particularly, the invention in its preferred embodiments includes in combination with an axial splitpin tumbler-type lock having a lock cylinder, a threepart plug assembly inserted in the cylinder, and a plurality of tumbler assemblies carried by the plug assembly, said plug assembly including a rear sleeve part fixed to the cylinder, a rotatable intermediate spindle part including a lock shaft extending through the sleeve part, a rotatable front drive part, and a post fixed to the drive part and having means drivingly engaging a key for rotation of the drive part by turning the key, said plug assembly parts each having a plurality of longitudinal bores arranged for alignment with the bores in the remaining parts in an initial disposition of the parts, said bores being asymmetrically disposed about the longitudinal axis of the plug assembly, whereby the dirve part may be rotated relative to the other parts and provide an inoperative or ineffectual second disposition of the parts wherein a blank area of the drive part is disposed over aligned bores in the spindle and sleeve parts and prevents access thereto, while at least one bore of the drive part is disposed in alignment with aligned bores in the spindle and sleeve parts, one of the tumbler assemblies being disposed in each series of aligned bores in said initial disposition of the parts and each tumbler assembly including spring means, a rear tumbler pin, an intermediate tumbler pin, and a front tumbler pin, said spring means yieldingly urging the several pins forwardly in the bores in said initial disposition of the parts, whereby a first interfacial plane between the sleeve and spindle parts, and a second interfacial plane between the spindle and drive parts are bridged by tumbler pins to secure the parts against relative rotation, and the tumbler pins in each series of aligned bores may be shifted longitudinally to cause the interface between the rear pin and the intermediate pin to coincide with the first plane while the second plane is bridged by at least one pin to thereby free the drive and spindle parts for cojoint rotation relative to the sleeve part, or alternatively to cause the interface between the intermediate pin and the front pin to coincide with the second plane while at least the interface between the aligned bores having the blank area thereover in said second disposition of the parts is bridged by one of said pins to thereby free the drive part for rotation relative to the spindle part and the sleeve part: first and second keys each adapted for drivingly engaging the post and having different bittings engaging the front tumbler pins, the bittings of the first key being adapted for shifting the tumbler pins longitudinally in each series of aligned bores so as to free the drive and spindle parts for cojoint rotation by the key from said initial disposition of the parts to operate the lock, the bittings of the second key being adapted for shifting the tumbler pins longitudinally in each series fo aligned bores so as to free only the drive part for rotation by the key from said initial disposition of the parts to said inoperative second disposition of the parts, and said second key being removable from the lock in said second disposition of the parts to leave the parts secured against relative rotation, whereby the lock is rendered inaccessible for direct picking and the second and first keys must be employed successively in that order to operate the lock.

In further preferred embodiments of the invention, means are provided on the lock cylinder for indicating when the second key is oriented in said second disposition of the parts. More preferably, the cylinder includes a retaining flange surrounding the post and having angularly spaced apart first and second notches, and each of the keys includes an outwardly projecting lug received in the first notch when its key is employed in the initial disposition of the parts and turning behind the flange to retain the key in the lock when turned, the lug of the second key being movable through the second notch for removing and reinserting the second key in the second disposition of the parts. Additional preferred structure is described hereinafter.

The lock and key combination of the invention provides a valuable increase in lock security while at the same time increasing the versatility of the lock. The lock itself is relatively simple and economical in its construction and operation, its preferred embodiment actually being simpler and less expensive than the preferred embodiment of the above-described Kerr patent lock, on which the present lock is based. The only additional part required is a second key, while parts of the preferred embodiment of the Kerr lock are obviated.

BRIEF DESCRIPTION OF THE DRAWINGS The attached drawings illustrate a preferred embodiment of the invention, without limitation thereto. In the drawings, like elements are identified by like reference symbols in each of the views, and:

FIGS. 1 and 2 are perspective views of first and second keys, respectively, which serve to operate the lock illustrated in FIG. 3, in accordance with the invention;

FIG. 3 is a front perspective view of a preferred embodiment of an axial split-pin tumbler-type lock employed in the invention illustrating the lock in its initial or starting attitude;

FIG. 4 is a front elevational view of the lock on a reduced scale;

FIG. 5 is a transverse sectional view of the lock on a reduced scale, taken on line 55 of FIG. 3, also showing an alternate position of the locking bolt in phantom lines;

FIG. 6 is an enlarged partly side elevational and partly longitudinal sectional view of the lock, with certain parts broken away and in section, the sections being taken aubstantially on lines 6-6 of FIG. 4, illustrating the lock with its plug assembly parts in an initial disposition thereof;

FIG. 7 is a fragmentary view of the lock similar to FIG. 6 but having the first key inserted therein, the key being shown fragmentarily and with parts broken away and in section, illustrating how the lock may be operated by turning the key;

FIG. 8, is a view similar to FIG. 7 but showing the second key inserted in place of the first key, with the key being shown fragmentarily and with parts broken away and in section, illustrating how one plug assembly part may be rotated by turning the key;

FIG. 9 is a view similar to FIG. 8 but with the key turned to place the lock in an inoperative second disposition of its plug assembly parts; and

FIG. 10 is a view similar to FIG. 9 but with the key removed to leave the plug assembly parts secured against relative rotation in their second disposition.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1-3 of the drawings, the lock and key combination of the invention includes a'first key 1, which may be termed a true operating key, a second key 2, which may be termed a lockoutor prevent key, and an axial split-pin tumbler-type lock 3. The keys are of generally conventional construction. Thus, the key 1 includes a wing-type handle 11 secured to a cylindrical tubular shank or bit 12. A series of partly cylindrical bittings 13 is provided on the outer periphery of the shank 12 at its circular free end 14. The bittings 13 in the illustrative preferred embodiment number seven, and they are angularly spaced around the end 14 on 45 centers, except for two bittings, which are spaced apart on 90 centers.

A rectangular retaining lug 15 is fixed on the outer surface of the key shank 12 closely adjacent to and spaced from the end 14, equidistantly between those bittings 13 which are spaced apart 90. On the inner surface of the shank l2, behind the lug 15, and likewise adjacent to and spaced from the free end 14, is a rectangular locating and plug-engaging lug 16. Similarly,

the second key 2 includes'a handle 21, a tubular shank 22, partly cylindrical bittings 23, a shank free end 24, a retaining lug 25, and a locating and plug-engaging lug 26. At least some of the bittings l3 and 23 on the respective keys 1 and 2 have different depths, for arranging the lock tumbler assemblies in different dispositions, as described hereinafter.

Referring to FIGS. 3-6, the lock 3 is based upon the lock disclosed in the above-identified Kerr US. Pat. No. 3,541,819. The lock 3 includes a lock cylinder 30, a three-part plug assembly 32 inserted in the cylinder, and a plurality of tumbler assemblies 34 carried by the plug assembly. A radial locking bolt 36 is carried by the plug assembly, and it is rotatable through an angle of 90, between the positions illustrated in full and phantom lines in FIG. 5, although any suitable degree of rotation may be selected.

The lock cylinder 30 includes a circular tubular threaded body 38 having flats 39 on opposite sides thereof, an enlarged ring-like and tapered head 40 integral with one end of the body 38, and an annular retaining flange 41 integral with the head 40 around the inner periphery thereof. The head 40 and the flange 41 form part of the face or front end of the lock 3. The flange 41 is provided with first and second rectangular, radial notches 42 and 43, respectively, and it defines the outer wall of a circular keyway 44 in the face of the lock 3. The notches 42 and 43 are disposed at angular distances between centers of 45. The cylinder body 38 has a bore 45 of slightly greater diameter than the internal diameter of the head 40, providing an annular shoulder 46 (FIG. 6) at the junction of the body and head within the cylinder 30.

A mounting nut 48, illustrated in FIG. 6, is in threaded engagement with the cylinder body 38, and serves for mounting the lock 3 upon a door or other suitable structure (not illustrated). Thus, the cylinder body 38 may be inserted through a suitable hole or opening in such a door, and a door panel may be clamped between the nut 48 and the cylinder head 40.

Referring particularly to FIG. 6, the plug assembly 32 includes a rear sleeve part 50 fixed to the cylinder body 38 by a set screw 52, a rotatable intermediate spindle part 54 and a rotatable front drive part 56. The several plug assembly parts 50, 54 and 56 are generally cylindrical and have external diameters approximating the diameter of the bore 45 of the cylinder body 38. The spindle and drive parts 54 and 56 are retained at the front end of the cylinder body 38 by the shoulder 46, on which the drive part 56 abuts.

The spindle part 54 includes an axial cylindrical lock shaft 60 extending rearwardly through and journaled in the sleeve part 50. The shaft terminates in a threaded end 62 having flats 64 on opposite sides thereof-and projecting rearwardly beyond the cylinder body 38. The spindle part 54 is provided with an axial cylindrical pilot bore 66 extending rearwardly from its front face.

A cylindrical stub shaft 68 extends axially rearwardly from the drive part 56, and a cylindrical center post 70 extends axially forwardly therefrom. A rectangular longitudinal lug-receiving groove 72 (FIGS. 3-5) is formed along the side of the post 70. The shaft 68 is received in the pilot bore 66 in the spindle part 54. The post 70 extends to the face of the lock, its outer end being substantially in a plane with the from surfaces of the head 40 and the retaining flange 41. The post 70 is spaced inwardly from the retaining flange 41, to form the annular keyway 44 therewith. In the illustrative embodiment, the stub shaft 68 and the post 70 are formed integrally with the drive part 56, but it may be preferable that the shaft and the post constitute a separate member or members fixed to the drive part 56 and be constructed of different material.

The sleeve part 50, the spindle part 54, and the drive part 56 of the plug assembly 32 each have a plurality of longitudinal bores 74, 76, and 78, respectively, arranged for alignment or registry with the bores in the remaining parts in an initial disposition of the parts, such as illustrated in FIG. 6. The bores 74 in the sleeve part 50 are blind bores, and the remaining bores 76 and 78 extend through their parts 54 and 56. The bores in each part are asymmetrically disposed about the longitudinal axis of the plug assembly 32. While another number of bores may be provided, the preferred illustrative embodiment has seven bores in each part. As illustrated in FIG. 5 for the bores 78 in the drive part 56, the bores in each part are arranged at intervals of 45, center-to-center, about the longitudinal axis of the plug assembly 32, and at an interval of 90 between two of them. The area 80 of the drive part 56 between'the latter bores constitutes a blank area thereof, and a similar blank area exists in the sleeve and spindle parts 50 and 54.

In the structure of the lock 3 so far described, the spindle part 54 is rotatable in the cylinder 30 for operation of the lock, in a manner such as described hereinafter. The drive part 56 also is rotatable in the cylinder 30. The spindle part 54 and the drive part 56 are rotatable relative to each other or cojointly. The freedom of the spindle and drive parts to rotate is controlled by the tumbler assemblies 34.

As illustrated in FIG. 6, each tumbler assembly 34 includes a coil compression spring 82, a rear tumbler pin 84, an intermediate tumbler pin 86, and a front tumbler pin 88, each of the pins being cylindrical. In each series of aligned bores 74, 76, and 78, a coil compression spring 82 is seated in the blind bore 74, av rear tumbler pin 84 is seated on the spring 82 in the blind bore 74, an intermediate tumbler pin 86 is inserted in the spindle part bore 76 and abuts on the rear pin 84, and a front tumbler pin 88 is inserted in the drive part bore 78 and abuts on the intermediate pin 86. While the several pins are, in general, associated with respective principal bores, as described, they are of varying lengths, which may exceed the lengths of the bores. The several pins in each tumbler assembly 34 are spring-pressed by their spring 82, to cause the front pins 88 to abut on the rear surface of the retaining flange 41, as seen in FIG. 6. A portion only, less than one-half, of each front pin 88 is exposed in the keyway 44 (see FIG. 4), and thereby accessible for engagement with a key.

In the foregoing initial disposition of the parts, the several pins bridge a first interfaeial plane 90 between the sleeve part 50 and the spindle part 54, and a second ,interfacial plane 92 between the spindle part 54 and the drive part 56, to secure the parts against relative rotation. The tumbler pins in each assembly 32 maybe shifted longitudinally against the pressure of the spring 82, moving the pins rearwardly, to cause the interface between the rear pin 84 and the intermediate pin 86 to coincide with the first plane 90, while the second plane 92 is bridged by at least one pin of the several plug assemblies 32, to thereby free the drive part 56 and the spindle part 54 for cojoint rotation relative to the sleeve part 50. Alternatively, the tumbler pins may be shifted longitudinally to cause the interface between the intermediate pin 86 and the front pin 88 to coincide with the second plane 92, while the first plane is bridged by at least one pin, as described hereinafter, to thereby free the drive part 56 for rotation relative to the spindle part 54 and the sleeve part 50.

The first key 1, illustrated in FIG. 1, is constructed for operating the lock 3. The free end 14 of the key shank 12 is inserted in the keyway 44, with the locating lug 16 entering the groove 72 in the post 70, thereby providing a driving interengagement of the key and the post 70, and also the drive part 56. The retaining lug 15 on the key enters the first notch 42 in the retaining flange 41. As illustrated in FIG. 7, when the key 1 is inserted until its free end 14 bottoms on the drive part 56, the key bittings l3 engage respective front tumbler pins 88 and shift the tumbler pins in the several tumbler assemblies 34 to positions such as illustrated. The retaining lug 15 on the key moves behind the retaining flange 41.

At this time, the interface between the rear pin 84 and the intermediate pin 86 in each of the tumbler assemblies 34 coincides with the first interfacial plane 90, while the second interfacial plane 92 is bridged by at least one tumbler pin. Any or all of the bridging pins may be intermediate pins 86 or front pins 88, and in FIG. 7, they are intermediate pins 86. The first key 1 may be rotated and, owing to the interengagement of its locating lug 16 with the center post 70, the drive part 56 rotates therewith. Further, owing to the bridging of the second interfacial plane 92 by pins such as the intermediate pins 86 seen in FIG. 7, the spindle part 54 also rotates with the key. Accordingly, the lock shaft 60 is rotated, to operate the lock.

One of various possible locking arrangements is illustrated in the drawings. Referring to FIG. 6, the locking bolt 36 is secured on the threaded end 62 of the lock shaft 60 by means of two nuts 94 and 96 on opposite sides of the bolt. The bolt then rotates with the lock shaft 60, and may be rotated between locking and unlocking positions, for example, as illustrated respectively by full and phantom lines in FIG. 5.

In order to limit the movement of the locking bolt 36 to a 90 throw, as in the illustrative embodiment, a stop flange 98 (FIG. 6) is formed on the lock cylinder body 38, integrally therewith, and projects rearwardly therefrom. A stop washer 100 is mounted on the lock shaft, adjacent to the cylinder body 38, and the washer is rotatable with the shaft. The stop washer 100 is contoured to abut on the stop flange 98 at the opposite ends ofa 90 path of travel, thereby limiting rotation of the lock shaft 60 to a selected 90 arc of the circle of rotation.

The operation of the lock 3 as so far described, and the corresponding structure, are essentially as described in the above-identified Kerr U.S. Pat. No. 3,541,819. The patent further describes the function of a front plug part numbered 18 like the drive part 56 herein, which serves to induce a false picking". The false picking is facilitated by tumbler balls 42 in the patent. In the present invention, inducement ot false picking is not relied on, although it may play a part in preventing unauthorized access. Therefore, the tumbler balls 42 of the preferred structure of the patent are omitted from the preferred structure of the present invention. Also omitted from the preferred structure of the patent are a plunger or limit pin 82, its spring 86, and a clearance notch 80 in the front plug part, which would interfere with the operation of the lock in the present invention.

The present invention achieves an additional function employing the structure including the drive part 56, and the second key 2, as illustrated in FIGS. 8-10, to which reference now is made. As in the case of the first key 1, the free end 24 of the shank 22 of the second key 2 is inserted in the lock keyway 44. The locating and plug-engaging lug 26 engages the center post 70 in its groove 72 (see FIG. 9), and the retaining lug 25 is received in the first notch 42. When the second key 2 is fully inserted, as illustrated in FIG. 8, its free end 24 bottoms on the drive part 56, and the retaining lug 25 is behind or inwardly of the retaining flange 41. The different bittings 23 of the second key 2 engage the front tumbler pins 88 to shift the pins in each tumbler assembly 34 longitudinally, to cause the interface between each intermediate pin 86 and adjacent front pin 38 to coincide with the second plane 92. At the same time, at least one of the tumbler pins of the tumbler assemblies 34 bridges the first interfacial plane 90, preventing the spindle part 54 from rotating. The drive part 56 thereby is freed for rotation relative to the spindle part 54 and the sleeve part 50. The second key 2 is rotated from the position illustrated in FIG. 8 to the position illustrated in FIG. 9, counterclockwise as viewed at the face of the lock, and through an angle of 45 degrees. The lock then is in a second disposition of the plug assembly parts, as illustrated in FIG. 9. At this time, the blank area 80 (see also FIG. of the drive part 56 is disposed over or in alignment with one pair of aligned bores 76 and 74 in the spindle part 54 and the sleeve part 50, respectively, preventing access to such bores. Six other pairs of aligned spindle and sleeve part bores 76 and 74 are aligned with respective bores 78 in the drive part 56, as illustrated by the lower series of aligned bores in, FIG. 9. The retaining lug 25 of the second key 2 then is in registry with the second notch 43 in the retaining flange 41. The second disposition of the plug assembly parts is an inoperative disposition, inasmuch as the locking function has not been affected, i.e., the locking bolt 36 is not moved from its original position, illustrated in FIGS. 3-6.

The second key 2 may be removed from the lock 3, with the retaining lug 25 on the key moving through the second notch 43 in the retaining flange 41, and it may be reinserted in the reverse manner. With the second key 2 removed, the tumbler assemblies 34 are disposed in the lock 3 as illustrated in FIG. one rear pin 84 and an adjacent intermediate pin 86, shown in the upper portion of the view, are urged by a spring 82 into abutting engagement of the intermediate pin 86 with the drive part 56 as in FIG. 9, while the intermediate pin 86 bridges the first plane 90 between the sleeve part 50 and the spindle part 54. It is preferred in addition that the first plane 90 be bridged by pins in some or all of the remaining series of aligned bores, as illustrated by the rear pin 84 in the bottom portion of FIG. 10. In this manner, the spindle part 54 is fixed or locked to the sleeve part 50, in turn fixed to the lock cylinder 30, so that the lock cannot be operated. The second interfacial plane 92 is bridged by one of the tumbler pins in each series of aligned bores around the plug assembly 32, although the second plane in other embodiments may be bridged by a lesser number of pins, as little as one. The bridging pin is an intermediate pin 86 as illustrated in the lower portion of FIG. 10, but alternatively, may be a front pin 88. With both interfacial planes 90 and 92 bridged by tumbler pins, the sleeve part 50, the spindle part 54, and the drive part 56 are secured against relative rotation.

The intermediate pin 86 and the adjacent rear pin 84 are rendered inaccessible by the disposition of the 'blank area of the drive part 56 thereover, so that the lock cannot be picked directly. The same result would be achieved if the first interfacial plane were bridged by the rear pin 84 rather than the intermediate pin 86. It is necessary to prevent picking that the first plane 90 be bridged by one or the other of the two pins 84 and 86 that are covered by the blank area 80.

The first key 1 is inoperative to function in any way when the parts are in their second disposition, as in FIG. 10, and, consequently, operation of the lock 3 by the first key 1 is locked out or prevented by the use of the second key 2. In order to operate the lock from the condition of FIG. 10, it is necessary initially to employ the second key 2 in a reverse manner, to return the plug assembly parts to their initial disposition, illustrated in FIG. 8, followed by removal of the second key. Then, the first key 1 must be inserted, as illus- .trated in FIG. 7, and rotated to operate the lock, as described above.

. In the illustrative preferred embodiment of the invention, the stop flange 98 and the stop washer 100 (FIG. 6) are constructed so as to limit the rotation of the spindle part 54 to clockwise rotation (as viewed from the front) through an angle of 90 from the initial disposition of the parts. The first key 1 is inserted for operation of the lock 3 with the parts disposed as illustrated in FIGS. 3 and 4, in which disposition the retaining lug 15 is inserted through the first notch 42. The first key 1 is rotated 90 in the clockwise direction, as governed .by the stop flange 98 and stop washer 100, to rotate the locking bolt 36 through the same are, is illustrated in FIG. 5, and thereby operate the lock. The first key 1 cannot be removed in its rotated position, inasmuch as itsretaining lug 15 is held in place by the retaining flange 41. The first key 1 can only be removed by returning it to its initial position, in which the parts are in their initial disposition, as in FIG. 3, when the retaining lug 15 can be withdrawn through the first notch 42. The lock and the first key 1 thus provide a one-way key pull, which requires the operator to return the key to its initial position in order to remove it from the lock, and insures that the lock is restored to its initial condition before the operator leaves. The second notch 43 is outside of the travel are of the retaining lug 15 of the first key 1. Consequently, the first key 1 cannot be removed by withdrawing the lug 15 through the second notch 43 in the second disposition of the parts, which would place the locking bolt 36 in an undesired position.

The second key 2, like the first key I, initially is inserted when the parts are in their initial disposition illustrated in FIGS. 3 and 4. To perform its function, the second key 2 is turned in the counterclockwise direction, until its retaining lug 25 registers with the second notch 43. The second notch 43 functions to indicate when the second key is oriented in the second disposition of the parts, and the notch also permits removal of the key. allowing the retaining lug 25 to pass therethrough. It will be apparent that the retaining lug 25 may be dispensed with, and other indicating means may be provided on the retaining flange 41 or the head 40, such as a suitable marking which is related to the position of the second key 2.

The lock 3 can be operated with the first key 1 alone, so long as it is in the condition illustrated in FIGS. 3-6. In this condition, however, it is subject to possible direct picking, rendered more difficult by the use of a three-part plug assembly 32. In order to prevent direct picking, the second key 2 is used, in the manner illustrated in FIGS. 8l0. The lock in the position of FIG. 10 then cannot be picked directly, because a picking tool cannot penetrate the blank area 80 of the drive part 56, behind which a tumbler pin 86 (or 84) bridges the first interfacial plane 90 and prevents the spindle part 54 from rotating. The only picking operation possible in this condition of the lock is to first pick the lock by moving the tumbler pins so as to free the drive part 56 for rotation back to the initial disposition of the parts, illustrated in FIG. 6. Then, it is necessary to pick the lock a second time and in such a manner as to free the spindle part 54 for rotation with the drive part 56, and not free only the drive part. The intricacy of the picking operations and the time consumed are prohibitive if, in fact, the picking can be accomplished by unauthorized persons. Once the second key 2 is used, therefore, two keys are required to operate the lock, and even the authorized possessor of a first key 1 is unable to open the lock without the second key 2. For different locks, the keys 1 and 2 can be provided with various arrangements of bittings l3 and 23, with correspondingly varying tumbler pin sizes. The bittings on either key may remain the same while the bittings on the other key and appropriate tumbler pin sizes are varied for different locks.

While a preferred embodiment of the invention has been illustrated and described, it will be apparent that various changes and modifications may be made therein within the spirit and scope of the invention. It is intended that such changes and modifications be included within the scope of the appended claims.

Having thus described the invention, what I claim as new and desire to secure by Letters Patent is:

1. In combination with an axial split-pin tumbler-type lock having a lock cylinder, a three-part plug assembly inserted in the cylinder, and a plurality of tumbler assemblies carried by the plug assembly, said plug assembly including a rear sleeve part fixed to the cylinder, a rotatable intermediate spindle part including a lock shaft extending through the sleeve part, a rotatable front drive part, and a post fixed to the drive part and having means drivingly engaging a key for rotation of the drive part by turning the key, said plug assembly parts each having a plurality of longitudinal bores arranged for alignment with the bores in the remaining parts in an initial disposition of said parts, said aligned bores being asymmetrically disposed about the longitudinal axis of the plug assembly, whereby the drive part may be rotated relative to the other parts and provide an inoperative second disposition of the parts wherein a blank area of the drive part is disposed over aligned bores in the spindle and sleeve parts and prevents access thereto, while at least one bore of the drive part is disposed in alignment with aligned bores in the spindle and sleeve parts, one of said tumbler assemblies being disposed in each series of aligned bores in said initial disposition of the parts and each tumbler assembly including spring means, a rear tumbler pin, an intermediate tumbler pin, and a front tumbler pin, said spring means yieldingly urging the several pins forwardly in the bores in said initial disposition of the parts, whereby a first interfacial plane between the sleeve and spindle parts, and a second interfacial plane between the spindle and drive parts are bridged by tumbler pins to secure the parts against relative rotation, and the tumbler pins in each series of aligned bores may be shifted longitudinally to cause the interface between the rear pin and the intermediate pin to coincide with said first plane while said second plane is bridged by at least one pin to thereby free the drive and spindle parts for conjoint rotation relative to the sleeve part, or alternatively to cause the interface between the intermediate pin and the front pin to coincide with said second plane while at least the interface between said aligned bores having said blank area thereover in said second disposition of the parts is bridged by one of said pins to thereby free the drive part for rotation relative to the spindle part and the sleeve part:

first and second keys each adapted for drivingly engaging said post and having different bittings engaging said front tumbler pins, the bittings of the first key being adapted for shifting the tumbler pins longitudinally in each series of aligned bores so as to free the drive and spindle parts for cojoint rotation by the key from said initial disposition of the parts to operate the lock, the bittings of the second key being adapted for shifting the tumbler pins longitudinally in each series of aligned bores so as to free only the drive part for rotation by the key from said initial disposition of the parts to said inoperative second disposition of the parts, and said second key being removable from the lock in said second disposition of the parts to leave the parts secured against relative rotation, whereby the lock is rendered inaccessible for direct picking and the second and first keys must be employed successively in that order to operate the lock.

2. A combination as defined in claim 1 and including means on said cylinder for indicating when said second key is oriented in said second disposition of the parts.

3. A combination as defined in claim 1 and wherein said cylinder includes a retaining flange surrounding said post and having angularly spaced apart first and second notches, and each of said keys includes an outwardly projecting lug received in said first notch when its key is employed in said initial disposition of the parts and turning behind said flange to retain the key in the lock when the key is turned, the lug on said second key being movable through said second notch for removing and reinserting the second key in said second disposition of the parts.

4. A combination as defined in claim 3 and including stop means interengaging said cylinder and said lock shaft for limiting said conjoint rotation of the drive and spindle parts in operation of the lock so that said second notch is outside of the travel are of the lug on said first key, thereby preventing removal of the first key in said second disposition of the parts.

5. A combination as defined in claim 3 wherein said bores in each part are seven in number arranged at intervals of 45 degrees about the longitudinal axis of the plug assembly and at an interval of between two of them, the area of said drive part between said latter bores comprising said blank area thereof. 

1. In combination with an axial split-pin tumbler-type lock having a lock cylinder, a three-part plug assembly inserted in the cylinder, and a plurality of tumbler assemblies carried by the plug assembly, said plug assembly including a rear sleeve part fixed to the cylinder, a rotatable intermediate spindle part including a lock shaft extending through the sleeve part, a rotatable front drive part, and a post fixed to the drive part and having means drivingly engaging a key for rotation of the drive part by turning the key, said plug assembly parts each having a plurality of longitudinal bores arranged for alignment with the bores in the remaining parts in an initial disposition of said parts, said aligned bores being asymmetrically disposed about the longitudinal axis of the plug assembly, whereby the drive part may be rotated relative to the other parts and provide an inoperative second disposition of the parts wherein a blank area of the drive part is disposed over aligned bores in the spindle and sleeve parts and prevents access thereto, while at least one bore of the drive part is disposed in alignment with aligned bores in the spindle and sleeve parts, one of said tumbler assemblies being disposed in each series of aligned bores in said initial disposition of the parts and each tumbler assembly including spring means, a rear tumbler pin, an intermediate tumbler pin, and a front tumbler pin, said spring means yieldingly urging the several pins forwardly in the bores in said initial disposition of the parts, whereby a first interfacial plane between the sleeve and spindle parts, and a second interfacial plane between the spindle and drive parts are bridged by tumbler pins to secure the parts against relative rotation, and the tumbler pins in each series of aligned bores may be shifted longitudinally to cause the interface between the rear pin and the intermediate pin to coincide with said first plane while said second plane is bridged by at least one pin to thereby free the drive and spindle parts for conjoint rotation relative to the sleeve part, or alternatively to cause the interface between the intermediate pin and the front pin to coincide with said second plane while at least the interface between said aligned bores having said blank area thereover in said second disposition of the parts is bridged by one of said pins to thereby free the drive part for rotation relative to the spindle part and the sleeve part: first and second keys each adapted for drivingly engaging said post and having different bittings engaging said front tumbler pins, the bittings of the first key being adapted for shifting the tumbler pins longitudinally in each series of aligned bores so as to free the drive and spindle parts for cojoint rotation by the key from said initial disposition of the parts to operate the lock, the bittings of the second key being adapted for shifting the tumbler pins longitudinally in each series of aligned bores so as to free only the drive part for rotation by the key from said initial disposition of the parts to said inoperative second disposition of the parts, and said second key being removable from the lock in said second disposition of the parts to leave the parts secured against relative rotation, whereby the lock is rendered inaccessible for direct picking and the second and first keys must be employed successively in that order to operate the lock.
 2. A combination as defined in claim 1 and including means on said cylinder for indicating when said second key is oriented in said second disposition of the parts.
 3. A combination as defined in claim 1 and wherein said cylinder includes a retaining flange surrounding said post and having angularly spaced apart first and second notches, and each of said keys includes an outwardly projecting lug received in said first notch when its key is employed in said initial disposition of the parts and turning behind said flange to retain the key in the lock when the key is turned, the lug on said second key being movable through said second notch for removing and reinserting the second key in said second disposition of the parts.
 4. A combination as defined in claim 3 and including stop means interengaging said cylinder and said lock shaft for limiting said conjoint roTation of the drive and spindle parts in operation of the lock so that said second notch is outside of the travel arc of the lug on said first key, thereby preventing removal of the first key in said second disposition of the parts.
 5. A combination as defined in claim 3 wherein said bores in each part are seven in number arranged at intervals of 45 degrees about the longitudinal axis of the plug assembly and at an interval of 90* between two of them, the area of said drive part between said latter bores comprising said blank area thereof. 